The present invention relates broadly to syringe assemblies and more specifically to a plunger rod and piston having novel locking means facilitating assembly easily and quickly with a minimum application force.
Typical syringe assemblies of the type to which the present invention relates include a piston or plunger which fits snugly in the barrel of the syringe and is adapted for actuation axially therein to aspirate and discharge contents through the hypodermic needle by means of a plunger rod attached to the piston. The piston and rod are separate elements having a suitable form of means for locking the parts together so they function as an integral unit.
Typical prior art techniques for attaching the plunger rod and piston are shown in U.S. Pat. Nos. 3,291,128, issued Dec. 13, 1966; 2,895,773 issued July 21, 1959 and 3,164,303 issued Jan. 5, 1965.
In U.S. Pat. No. 3,291,128, the plunger rod has a cylindrical tip with a plurality of radially extending, circumferentially spaced lugs and the piston has a pocket of an interior configuration corresponding to the plunger tip. The opening to the pocket is of a diametral dimension of substantially the same size as the cylindrical tip. Thus, a substantial force is required to press the tip in place since the lugs must be forced through a relatively thick walled narrow throat area of the piston. Furthermore, the parts have to be concentrically aligned before assembly and thus assembly by automatic equipment is practically impossible. The principal form of plunger rod and piston of U.S. Pat. No. 2,895,773 is basically the same as that described above. In addition, there are shown expansion lock arrangements which require post assembly manipulation to lock the parts together. U.S. Pat. No. 3,164,303 shows a screw threaded arrangement.
In syringe assemblies where the plunger and piston rod are assembled by the manufacturer, it has been found that the units mentioned above are only capable of being assembled by hand and do not lend themselves to faster, automated machine assembly techniques. In disposable syringes, the plunger is generally inserted into the barrel and the plunger rod maintained as a separate element until it is desired to activate the syringe. In disposable syringe assemblies, the forward end of the barrel usually has some form of diaphragm means or a cap closing the discharge end of the barrel. Presently in most of these disposable syringes, the plunger rod and piston are assembled by hand, which of course is time consuming and increases the cost of the overall assembly. These disposable syringe assemblies are usually housed in a somewhat flexible and pliable plastic package. This package is more elongated than if the plunger rod were included in the package as a separate item, and then assembled by the user when it is desired to use the syringe and this adds to the cost of manufacturing and shipping. Moreover there is also the danger of accidentally moving the plunger rod axially during packaging the assembly or in shipment which may prematurely rupture the diaphragm or displace the closure cap at the discharge end of the barrel. In some instances, the plunger rod is a separable element and in preparing the syringe for use, the user assembles the plunger rod to the piston, removes the closure cap and applies the needle to the end of the barrel. Some difficulties have been encountered in assembling the plunger rod by reason of the large axial force needed to seat the plunger rod in the piston in these prior assemblies. In some cases the force needed to assemble has been so great as to either burst the diaphragm prematurely or to create a sufficient internal pressure to blow the closure cap off the end of the barrel.
Accordingly, it is an object of the present invention to provide a new and useful plunger rod and piston including a novel locking arrangement facilitating assembly manually or by automatic machine techniques.
It is also an object of the invention to provide such an assembly wherein the parts assemble easily with a minimum force by a snap action and wherein the rod and piston are firmly secured together.
Another object is to provide such an assembly which is simple and inexpensive.